1. Field of the Invention
The present invention relates to a charging and discharging control circuit and a charging type power supply device of a secondary battery equipped with the charging and discharging control circuit. In particular, the present invention relates to a method of switching into and out of a testing state for evaluating the characteristics of the charging and discharging control circuit.
2. Description of the Related Art
The features of a lithium ion secondary battery which has greatly contributed to the popularization of mobile devices typified by a mobile telephone and a PHS are its small size, lightweight, and large capacity. Those features have led to realization of long time driving and lightness of the mobile devices. However, since a secondary battery is repeatedly charged and discharged, there is a high probability that the secondary battery will attain an overcharge state or an overdischarge state. If the secondary battery attains the overcharge state, the battery temperature rises so that the internal pressure of the secondary battery is increased and the metal Li is precipitated owing to generation of gas due to the decomposition of an electrolyte. Thus, there is a risk of ignition or explosion of the battery. On the other hand, if the secondary battery attains the overdischarge state, the electrolyte is decomposed to deteriorate the characteristics of the battery. In order to prevent such situations from occurring, a charging and discharging circuit is incorporated in a charging type power supply device.
A basic technique used by a charging and discharging control circuit involves providing a charging and discharging control switch circuit in a charging and discharging pathway, between a secondary battery and a mobile device. The charging and discharging control circuit detects abnormal states, such as an overcharge state in which the secondary battery is charged to a voltage level equal to or greater than a predetermined voltage, an overdischarge state in which the secondary battery is discharged to a voltage level equal to or less than a predetermined voltage, and an over-current state in which the secondary battery is discharged by an excess current. The charging and discharging control switch that opens and closes the current pathway of the charging type power supply device is turned off when an abnormal state is detected, thus preventing overcharge, overdischarge, and over-current states.
A lithium ion secondary battery has high internal impedance, and the battery voltage can consequently be seen to change according to charging and discharging currents. The battery voltage can be seen to be high when a charging current is flowing, while the battery voltage can be seen to be low when a discharge voltage is flowing. It is thus necessary to provide a delay time for detecting the abnormal states such as overcharge and overdischarge. However, the delay time causes extra time to be required to test the charging and discharging control circuit during an inspection process.
In particular, the delay time for detecting overcharge is generally set to a value on the order of several seconds. It is thus necessary to provide a testing state for the charging and discharging control circuit that shortens the delay time in order to shorten the testing time.
A method of controlling a delay circuit by adding one input terminal to the charging and discharging control circuit, and a method of initiating a testing state by applying a high voltage to a connection terminal of a charging device, exist for the testing state of the charging and discharging control circuit.
FIG. 2 shows a conventional charging and discharging control circuit, and discloses a case of applying a voltage equal to or greater than a prescribed voltage, the voltage being higher than that applied during normal operation, to the connection terminal of the charging device.
When an overcharge state occurs in a normal operating state, output from an overcharge detection comparator 213 becomes high level, and an internal portion control circuit 220 outputs a control signal to an internal portion delay circuit 221. The internal portion delay circuit 221 outputs a signal that controls a switching circuit 202 after a delay time t1 prescribed by using the output voltage as an input signal.
When a voltage equal to or greater than the high prescribed voltage is applied to the connection terminal of the charging device, and the voltage of the over-current detection terminal increases to be equal to or greater than the prescribed voltage, output from a Voltage detection comparator 215 becomes a high level. At this point the internal portion control circuit 220 is placed in a testing state to output a control signal that shortens the delay time of the internal portion delay circuit 221.
When an overcharge state occurs in the testing state, output from the overcharge detection comparator 213 becomes a high level, and the internal portion control circuit 220 outputs a control signal to the internal portion delay circuit 221. After a delay time t2 shortened by using the output voltage as an input signal, the internal portion delay circuit 221 outputs a signal that controls the switching circuit 202.
Problems like those described below exist with the conventional technique described above.
A very large number of the charging and discharging control circuits packed into a small size package are utilized. Accordingly, adding external portion terminals in order to control the detection delay time during testing invites cost increases, and this is not received well by the marketplace. Achieving this type of control function by using a small number of external portion terminals is therefore a large problem.
On the other hand, with a method that utilizes the over-current detection terminal, it is necessary to add a circuit in order to divide the voltage of the over-current detection terminal into a plurality of levels and detect the levels. In particular, the circuit structure becomes complex with the technique described above when a plurality of levels of over-current detection are required, and there is a problem in that it is difficult to ensure stable operation.
It is necessary to perform initial measurement during manufacturing of the charging and discharging control circuit in order to trim voltages used for detecting and releasing overcharge and overdischarge to a set voltage. When doing so, it becomes necessary to wait for a period of time equal to or greater than several seconds of the delay time when adding voltage in steps input to the charging and discharging control circuit. Assuming that the detection voltage is measured in 25 steps, and the delay time is set to 5 seconds, the amount of time needed to measure the overcharge detection voltage becomes 125 seconds. Even if a testing state that shortens the delay time to 1/50 of the normal delay time, for example, is used, 2.5 seconds will still be required per chip. This is a serious problem that invites increased testing costs during a manufacturing process.
In other words, it is necessary to further shorten the detection delay time in a manufacturing facility. It is also necessary to have both a state in which there is a delay time during normal operations, and a testing state having a short delay time, for customer IC evaluation and the like.